The invention is related to the field of energy harvesting, and in particular to a thermoelectric energy harvesting system using low voltages to efficiently harvest energy.
Energy harvesting is an area of growing importance because of its potential to reduce the usage of batteries in portable electronic devices. Ambient thermal energy can be harvested using thermoelectric means. FIG. 1 shows a typical thermoelectric harvester 2 using temperature differences to produce electrical energy and vice-versa. It can be modeled as a voltage source VT in series with resistance RT.
The open-circuit voltage output VT of the harvester 2 is proportional to the temperature difference defined as:VT=SΔT  (1)where S is the Seebeck coefficient of the material and ΔT is the temperature difference on the thermoelectric device. For most commercial thermoelectric harvesters, the Seebeck coefficient S is in the order of 10-30 mV/K. The harvester 2 can be used to extract electrical power from places with very little temperature difference but the output voltage that can be obtained from the harvester 2 might be as low as 25-50 mV.
The electrical circuit that interfaces with the thermoelectric harvester must be able to operate from this extremely low voltage. Also, batteries must be avoided to reduce cost. This issue presents a problem of how to startup from very low voltages and transfer energy to circuits that are powered by the thermoelectric harvester 2.
FIG. 2 shows a known circuit 4 used to startup from low voltages. In this circuit 4, the essential components are the coupled inductors being coupled to an always-on junction FET (JFET) such as M1. If a small voltage is present at the terminal VTH of the voltage converter, current flows through the primary winding L1 of the transformer TR1. The current is associated with an increasing exponential function and the voltage decreases with the same exponential function. The change of this current is positive, which lead to a positive voltage being induced in the secondary winding L2. The positive terminal of the inductor L2, which is connected to the gate of M1, is driven to a fixed voltage level by the diode of the JFET M1. Also, the negative terminal is shifted to a negative voltage level, charging the capacitor C2 to a negative level. When the current in the primary winding L1 saturates, the deviation and the induced voltage in the secondary winding L2 is zero, which leads to a drop in the secondary voltage.
Eventually, the transistor M1 switches off because the sum of the voltage of capacitor C2 and the secondary winding L2 becomes negative. This leads to the current through the primary inductor to decrease and a positive voltage is induced in the primary winding L1, delaying the current decrease. The output capacitor COUT is charged via diode D1 because the transistor M1 has a high resistance. The primary current reaches zero which results in the induced voltage in L2 becoming zero and C2 being discharged by R1 to the level of the input voltage. Thus the JFET M1 starts conducting again and the operation cycle repeats.
However, the circuit 4 requires a transformer with a large turns ratio to achieve startup from low voltages. In addition, the JFET transistor M1 is required to be normally ON to supply a large current with 0 VGS and very low VDS (˜25 mV). This is extremely difficult to achieve startup using this circuit and not suitable for use with portable devices.